Brent : thanks for the detailed explanation. As for your suggestion, I'm
trying to go the other way around: Postgis --> CSV --> QGIS.
The goal is to quickly extract data from a very large table in Postgis
using an extent drawn by the user on the canvas. So I do a `COPY (SELECT
... FROM ... WHERE ST_Intersects(geometry, extent_drawn_on_canvas)) TO
...` to get a CSV file, and then I load the CSV file into QGIS,
expecting to get the exact same values than in Postgis (NULL when NULL
in Postgis, empty string when empty string in Postgis).
Richard: nice workaround, and it almost works! With a space in double
quotes (" "), I get in my QGIS 3.12 a single space string, not an empty
string, even when Trim fields in checked. But with a space and no
quotes, it works! That is just like the following.
col1,col2,col3,col4
1,, ,"foo"
col3 get an empty string (if Trim fields is checked).
Hope this is a stable behavior.
Thanks again!
Regards.
Yann
Le 15/04/2020 à 12:35, brent.w...@niwa.co.nz a écrit :
Not that I'm aware of -
Text files do not have an implicit way of representing null values, except for
an empty string.
Databases do this better, with an isnull() function which is different to an
='' (2x single quotes) expression - the empty string.
Some software packages use a locally specified string (eg: NA) to represent
nulls, so you can use an !='NA' expression to exclude nulls.
The ^A (control-A) ANSI character also represents a null, but good luck getting
a useful text file containing control codes. Generally non-trivial.
Note that an "=null" expression is semantically void, null means unknown, so
two nulls do not equal each other, we don't know what they equal by definition.
Which is why SQL has the is null/isnull() approach, it is not a logical "="
operator.
I suggest you either load your text file into a database table (spatialite or
postgis?), define the nulls as db null values (which are NOT the same as empty
strings)
& QGIS can use the db to do the null operations, or alternatively you could define a
string (not an empty string) that is used to represent nulls in your file, and use an
"=" operator. For numerical values you can use a value which does not occur in the
dataset to represent a null .
Brent Wood
Programme leader: Environmental Information Delivery
NIWA
DDI: +64 (4) 3860529
________________________________________
From: Qgis-user <qgis-user-boun...@lists.osgeo.org> on behalf of Yann Voté
<ygver...@lilo.org>
Sent: Wednesday, April 15, 2020 21:57
To: qgis-user@lists.osgeo.org
Subject: [Qgis-user] Distincguis NULL and empty strings when reading CSV
files
Hi all,
Just wondering if there is a way to distinguish null values and empty
strings when loading CSV files in QGIS.
What I tried so far is quoting empty strings like in the following example.
col1,col2,col3
1,,""
I was expecting col2 to be NULL and col3 to be an empty string. But
actual result is that both col2 and col3 are NULL.
Is there another way ? Am I missing something ?
Thanks in advance.
Yann
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